Since polymers deteriorate due to heat or light, a phenolic antioxidant, a phosphoric antioxidant, a sulfuric antioxidant, a hydroxylamine compound, a hindered amine compound, a UV absorber, an acid scavenger, and others are generally added to the polymers.
For polyolefin polymerization reaction, Ziegler-Natta catalysts or metallocene catalysts are used, and in Patent Document 1, a so-called third-generation Ziegler-Natta catalyst has been proposed. After this proposal, as the development of catalysts proceeds, so-called fourth-generation and fifth-generation Ziegler-Natta catalysts as disclosed in Patent Document 2 and Patent Document 3, and a metallocene catalyst as disclosed in Patent Document 4 have been proposed. Conventionally, various kinds of stabilizers have been blended with a polymerized product and uniformly dispersed into it by melt-kneading and other processes to attain the long-term stabilization of polymers.
However, so as to achieve the stabilization, enormous energy is inefficiently consumed for blending various kinds of stabilizers by way of melt-kneading after polymerization. This energy is considered to be equal to amount to one third of the energy used for propylene production. Further in some cases, in order to address the defective dispersion of the stabilizers, a larger amount of stabilizers than necessary is required to be blended.
Among polymerization methods for producing a granular polymer product directly, a process for uniformly dispersing the stabilizers and an elimination of the kneading step by way of blending the stabilizers before or during polymerization has been proposed in Patent Documents 5 to 9 and others.
Patent Document 5 discloses a process for adding a hindered amine compound during polymerization or in the last phase of polymerization of an olefin polymer using a Ziegler-Natta catalyst, and shows that a more excellent stabilizing effect can be attained as compared with the process for adding a phenolic antioxidant on polymerization.
Patent Document 6 discloses a process of a-olefin polymerization in the presence of a phosphoric antioxidant, and shows that a more excellent stabilizing effect can be attained as compared with the process of post-addition because the antioxidant can be bonded to the inside of polymer particles homogeneously and uniformly, and that no extruder is needed for mixing the antioxidant.
Patent Document 7 discloses a process for using a hindered amine compound and a phosphoric antioxidant in combination in an olefin polymer polymerization system, and shows that a more excellent stabilizing effect can be attained as compared with the process of using solely the hindered amine compound.
Patent Document 8 discloses a process for using a catalyst having a specific ether compound as an internal electron donor, and shows that an excellent stabilizing effect can be attained when a phenolic antioxidant is used on polymerization and that the polymerization catalyst used is hardly decreased in activity and the resulting polymer product is hardly colored.
Patent Document 9 discloses a process of using a specific phosphoric antioxidant on polymerization, and shows that a polymer product having an excellent property against coloring in contact with water can be attained without any inhibition on olefin polymerization.
As is clear from the aforementioned related arts, adding stabilizers on polymerization in fact has an advantage of eliminating the blending step of the stabilizers in the post-polymerization process, while on the other hand, has disadvantages such as lowering the polymerization catalyst activity and coloring the polymer product by an interaction between the catalyst metal and stabilizers. Therefore, addition during polymerization is required, and there have been problems that polymerization conditions and controls become complicated. Particularly for continuous polymerization, an extra reactor has been used for preliminary polymerization in conventional production plants.
As to the phenolic antioxidant, Patent Document 8 discloses that phenolic antioxidants conventionally used for polyolefin including tetrakis(3-(3,5-ditert-butyl-4-hydroxyphenyl)propionyloxymethyl)methane and the like are not suitable for addition before polymerization because they lower catalyst activity.
There are various kinds of processes for polyolefin polymerization including gas-phase polymerization, slurry polymerization and the like. For adding to the polymerization system before the polymerization, a stabilizer that inhibits polymerization when it is used alone is required to be treated with a masking agent before addition, and the masking agent is required to be eliminated with the help of water or the like when the catalyst is deactivated. However, the treatment with a masking agent such as an organic aluminum generally requires a large amount of an aliphatic solvent, so that a stabilizer having subjected to such a treatment is not suitable for gas-phase polymerization.
Patent Document 1: Japanese Patent Laid-Open Publication No. S50-108385 (particularly, in claims)
Patent Document 2: Japanese Patent Laid-Open Publication No. S57-63310 (particularly, in claims)
Patent Document 3: Japanese Patent Laid-Open Publication No. H02-255810 (particularly, in claims)
Patent Document 4: Japanese Patent Laid-Open Publication No. S61-130314 (particularly, in claims)
Patent Document 5: Japanese Patent Laid-Open Publication No. S61-181803 (particularly, in claims)
Patent Document 6: Japanese Patent Laid-Open Publication No. S63-92613 (particularly, in claims)
Patent Document 7: Japanese Patent Laid-Open Publication No. H02-53804 (particularly, in claims)
Patent Document 8: Japanese Patent Laid-Open Publication No. H05-271335 (particularly, in claims and comparative example 1)
Patent Document 9: Japanese Patent Laid-Open Publication No. H08-208731 (particularly, in claims)